TANKHOUSE CONVERSIONS

Transcription

1 TANKHOUSE CONVERSIONS

2 Variations in reserve type, process economics and project viability can produce interesting challenges for the copper producer. Situations are more frequently arising where copper smelters may shut down for environmental compliance cost or feedstock shortage leaving a redundant electrorefinery. Such facilities can be converted fairly readily to copper electrowinning activities.this is especially attractive if some form of leaching and solvent extraction purification can be considered upstream at a location near enough to be a feasible source of electrolyte.. Faradelk has carried out studies for such conversions for plants in Democratic Republic of Congo and Canada. One of these is now proceeding as a project. Similarly as a leachable resource is used up, especially in heavily industrialized countries, it may be attractive to take a disused electrowinning plant formerly used in a L/SX/EW role and convert it to electrorefining to process copper scrap for example. ELECTROREFINING (ER) TO ELECTROWINNING (EW) This conversion can be viewed in three ways - Restricting the size of the new EW plant to the number of cells accommodated by the existing ER rectifier voltage output -Maintaining the circuit current of the existing ER plant rectifiers and the same total number of cells. Thus the rectifiers will need modification to suit the new voltages required but the busbar sections would be unchanged. - Adding rectifier current capacity to reach typical modern EW current densities over the existing total number of cells. This means that both rectifiers and busbars require modification.. Existing ER equipment that could be re-used Titanium starting sheet deposition blanks Starting sheet stripping and assembly machines Cathode washing machine Cells Cathode blanks Cathode washing and stripping machine Busbars (trunk and intercell) Tankhouse cranes Rectifiers (with transformer modifications) Electrolyte circulation tanks, pumps and heat exchangers Lead anodes from decopperizing/liberator cells

3 Modifications that would be needed In order to convert the ER facility into an EW facility, it will be necessary to make some piping modifications, change the cell-top furniture, upgrade the rectifiers for the increased voltage required and purchase the lead anodes needed for the cells. For SX fed electrolytes current densities of 320 A/m2 are now common ( Spence in Chile operates at 400+ A/m2 using air sparged cells. These densities pose extra problems for acid mist control.) The SX step and small transfer of iron with the copper means that ferric iron is easily controlled by a small bleed and current efficiency can be expected to be over 90%. Consequently, as mentioned above, the converted plant output could be raised using higher current density with slightly larger modifications to the rectifier thyristors and transformers and adding additional leaves to the circuit busbars. The bleed mentioned above arises from maintaining the electrolyte at under 1.5g/L ferric iron. This iron is transferred to the electrolyte by minor coextraction with copper ( of the order of 2000:1 ) and some transfer by aqueous entrainment of leachate in the loaded organic to stripping. In several modern L/SX/EW installations this bleed is run through an EMEW cell installation prior to the usual return to the leaching step. This reduces the circulating copper load, produces very high grade cathode down to <1g/LCu, regenerates sulphuric acid and so reduces the acid purchase requirement.. Conventionally copper refinery cells are arranged in half sections with current passing down one half section and then transferring to the adjacent half section and returning to the end of that half section near the cell where the current first entered. The two half sections thus make up a section. Cell harvesting is usually done by shorting out the section by a shorting switch which connects the busbar feeding the first half section with that taking the return current nearby on the second half section. Some ER plants use a variation on this design where long full half section lengths of full size busbar are used with shorting switches to enable shorting out of individual half sections so improving time efficiency. These long full busbar lengths would become redundant after the change to electrowinning and can used to supply full current bar for any additional busbar required. Although having the multiple sections arranged on a circuit as used in electrorefining is not ideal for electrowinning, the usual ER cell / busbar configuration will be acceptable provided the feed and return piping manifolds for each section enter to begin distribution to the two half sections through the back busbar for the section where the voltage difference between the cells is least. The main feed piping headers from the circulation pumps should be studied to avoid linking high potential difference areas of the circuit by short electroyte filled piping. The higher voltages involved will often mean the cell circuits must be split and more rectifiers added to achieve acceptable total voltages on the circuit for operator safety considerations. In order to convert the ER facility into an EW facility,. the modifications include, but are not necessarily limited to,:

4 a. Piping and pumping arrangements - The cell sections are fed in electrorefining by a common manifold feeding the half sections on each side. The half sections are relatively short and the cell voltages low, so the potential difference between the cells at the current feed and discharge ends of the section is small, and stray current loss across the electrolyte feed line is minimal. However, in electrowinning, the cell voltages are eight times higher and the cell feeds of electrolyte to each half section must be kept separated and enter the pair of half sections at the null point for the section to avoid stray current losses between the extreme cells of the section i.e. at the high voltage difference end of the section.. Modern practice is to feed EW cells using a sparging ring manifold on the cell floor which feeds the incoming electrolyte up between the electrodes. The use of this ring means the cell can be fed from either end.this can be a key point in a successful conversion. The cell half sections in the electrorefinery are usually located close to each other with only space for the single manifold feeding both half sections. Thus in most instances to convert the refinery piping It is simpler to use cell feed rings which can be fed from the walkway end adjacent to the cell overflow boxes. The existing ER feed manifolds would be disconnected. New feed manifolds for each half section would be run beneath the operating floor adjacent to the collection manifold for the electrolyte leaving the cells. The specific cell flows in EW are usually higher than ER, thus additional pumps and larger cell collection manifolds may be required. The downcomers leaving the cell overflow boxes may need to be bigger to suit a larger cell flow and the discharge hole in the overflow box may need to be enlarged. If there is sufficient space to run these two separate feed manifolds down between the half sections the existing cell feed lines ( with greater diameter) can be reused with these augmented by a sparge line down the centre of the floor of each cell.the ring or sparge line distributors will assist flow distribution between the electrodes and will reduce the effect of the cell flowrate constraint which results from the smaller diameter overflow box downcomer in the electrorefining cells than desired for electrowinning. To avoid such major modifications as changing the outlet downcomer diameter, the cell flow and deposition can be reviewed using the maximum flow that can be handled by the existing overflow box downcomers. The existing downcomer pipe will not operate as a free flowing downcomer at the greater flows needed and some depth of leaving electrolyte will be above it. This will result in some gulping by the leaving flow. Provision of a cover plate over the overflow box will reduce splashing inconvenience.. b. Cell-top furniture - Insulator spacing blocks (cell-top furniture) will need to be provided. In EW these are more sophisticated and are more precise locators than in ER. An alternative is to use clip on plastic locators on the lead anode bars c. Rectifiers Depending on the conversion approach chosen, these will need to be upgraded for the increased voltage required. This increased voltage is about 8-times higher in EW than in ER as noted above. Note if the thyristors of the original rectifiers can accept the higher voltage, only the transformers will need to be modified. As mentioned above the

5 circuit voltage limits for safe operation may require additional rectifiers on a larger number of smaller circuits d. Cranes- Generally the ER cranes will be of adequate capacity for EW duties. The crane lifting bales will need modification to suit lifting every third or second cathode as cell pulling in EW is done live.. Additional to the modifications will be inspection and maintenance of the electrode equipment and cranes by the manufacturers so that any items that may have deteriorated may be replaced. Insulation at crane hooks, trolley truck wheels and end trucks is required for electrical safety in most EW situations.. e. Cathodes There are two options relating to the cathodes, viz. If the existing ER plant uses starting sheets this system could be maintained. This would allow continued use of starting sheet mother blanks ( depositing electrowon sheets ), sheet stripping and fabricating equipment and cathode washing machines.. This option does restrict the current densities that can be used Change to permanent cathode technology If the change to permanent cathode technology was made, it would be necessary to install cathode washing and stripping machines Additional equipment required The major purchase is the complement of insoluble lead/calcium/tin anodes to fill all the cells. If it is desired to change from starting sheet operation to permanent cathode blanks a cathode washing and stripping machine would be needed plus the blanks themselves.. Acid mist generation in EW will need to be controlled and removed from the tankhouse atmosphere. Additional ventilation will be needed. The extent of this will be largely governed by the climatic situation and environmental workplace regulations. A minimum of floating ball blankets, interstitial foam and cell cover blankets with some form of mechanical ventilation should be allowed. Recommendations for the conversion from ER to EW The use of starting sheets is the cheapest option if the titanium blanks and original machinery are still available. The main benefits of converting an ER tankhouse into an EW circuit is that copper production can be maintained with minimal investment. Adequate copper leachate and solvent extraction capacity must be available upstream to deliver the strong electrolyte at the copper and acid tenors and at the flowrates required

6 Full use can be made of the converted refinery. All copper is produced at LME Grade A; probably 99.99% copper A large SX fed EW facility will be obtained for approx 50% of the cost compared to the new as built cost. (for an tpy EW plant as part of a SX/EW plant US$27 million c.f. approx US$ 60 million) ELECTROWINNING (EW) TO ELECTROREFINING (ER) This conversion is simpler electrochemically than the ER to EW conversion. Major concerns are the structural strength of the cells, crane capacities, handling of greater volumes of high value anode slimes, anode handling and casting machinery, liberator cells and arsenic /arsine control Existing EW equipment that could be re-used Titanium starting sheet deposition blanks Starting sheet stripping and assembly machines Cathode washing machine Cells ( with structural checks ) Cathode blanks Cathode washing and stripping machine Busbars (trunk and intercell) Shorting frame Tankhouse cranes ( with split anode loads ) Rectifiers Electrolyte circulation tanks, pumps and heat exchangers Lead anodes for decopperizing/liberator cells Modifications that Would be Needed In order to convert the EW facility into an ER facility, it will be necessary to make some piping modifications, change the cell-top furniture, and purchase the anode casting and handling equipment. Conventionally copper refinery cells are arranged in half sections with current passing down one half section and then transferring to the adjacent half section and returning to the end of that half section near the cell where the current first entered. The two half sections thus make up a section. Cell harvesting is usually done by shorting out the section by a shorting switch which connects the busbar feeding the first half section with that taking the return current nearby on the second half section.. In contrast the existing EW plant is likely laid out as one large section with a long tier of many cells down one side and a second tier connected to the first by a back busbar. Pulling cathodes and replacing copper anode scrap by fresh anodes in such an arrangement

7 requires a different technique to that usually found in ER plants. There are no small sections that can be locked out of circuit using shorting switches and cell electrode manipulating operations will have to be done live using the EW plant shorting frame. Such frames with an on load shorting switch may even result in higher time efficiency in the modified EW tankhouse than in a conventionally laid out refinery In order to convert the EW facility into an ER facility. the modifications include, but are not necessarily limited to,: a. Piping and pumping arrangements The electrolyte feed and removal piping of the earlier EW plant will be quite adequate for the ER operation.. Modern practice is to feed EW cells using a sparging ring manifold on the cell floor which feeds the incoming electrolyte up between the electrodes. The use of such a distributor on the cell floor may not be feasible with the slimes fall in an ER cell and a simple feed pipe at the opposite end of the cell from the overflow box may be more suitable. ( This could of course be fed from the EW cell feed manifold if this is adjacent to the overflow box. The specific cell flows in EW are usually higher than ER, thus existing pumps and cell collection manifolds should be suitable in the ER operation. b. Cell-top furniture - Insulator spacing blocks (cell-top furniture) will need to be reviewed. In EW these are more sophisticated and are more precise locators than in ER. They may need wider tolerances to suit the variability of cast copper anode lugs and tougher materials to accommodate the heavier cast anode weight. c. Rectifiers The EW rectifiers should be acceptable in both current and voltage output. The transformer tap changer may need recalibration to suit the lower output operating voltage required in ER d. Cranes- Generally the EW cranes will be of inadequate capacity for ER duties. The crane lifting bales will need modification to suit lifting every cathode and anode as cell pulling in ER is done dead.. Additional to the modifications will be inspection and maintenance of the electrode equipment and cranes by the manufacturers so that any items that may have deteriorated may be replaced. Insulation at crane hooks, trolley truck wheels and end trucks is required for electrical safety in most EW situations and will be suitable for ER duties. The crane Safe Working Load will be less than that needed for a full cell load of fresh cast copper anodes. Simple replacement of the cranes with heavier capacity units, in addition to being expensive in itself,can also affect the building structural design. These costs can be avoided if the EW cranes are used to lift only approx. half the anodes in the cell and the additional journeys in cell change out are accepted. Removing only two or three cells at a time for harvesting from the circuit using the shorting frame will likely counterbalance any adverse affects on time efficiency. Moreover if the EW lifting format was kept of lifting every second cathode the removal and replacement of cathodes on the first pull opposite fresh anodes could be done live which would again improve time efficiency..

8 e. Cathodes There are two options relating to the cathodes, viz. If the existing EW plant uses starting sheets this system could be maintained. This would allow continued use of starting sheet mother blanks (depositing electrorefined sheets ), sheet stripping and fabricating equipment and cathode washing machines.. This option does restrict the current densities that can be used as the Stripper cells are in the Commercial electrical circuit Change to permanent cathode technology If the change to permanent cathode technology was made, it would be necessary to install cathode washing and stripping machines as well as purchase the blanks themselves. f. Tankhouse ventilation - Control of acid mist generation in EW will have produced a well ventilated building. Such ventilation should be more than sufficient for working atmosphere temperature and humidity control in the ER operation. g. Cells The EW cells will need to be checked structurally to ensure they can carry the weight of a load of cast copper anodes at the higher ER cell temperature ( 65 deg C c.f. 50 deg C in EW ). If the cell walls are inadequate the number of anodes/cathodes per cell can be reduced and perhaps a wider inter electrode spacing used. This will impact the potential plant production. h. Slimes handling The ER process will produce significant amounts of valuable anode slime in the floor of the cell. This must be removed from the cell after the last cathode cycle when anode scrap is removed at the end of the mud run The lead anode slime handling scheme of the EW plant may be reutilized for ER slimes handling in a small operation. On a larger scale a portable slurry pump which can be lowered into the cell being cleaned plus slimes filtration equipment will be required. Alternatively, this pump could be connected to the original lead slime removal outlet in the cell. Additional Equipment Required The major purchases are the anode casting, anode preparation and anode scrap handling machines. If it is desired to change from starting sheet operation to permanent cathode blanks a cathode washing and stripping machine would be needed plus the blanks themselves.. Recommendations for the Conversion from EW to ER The use of starting sheets is the cheapest option if the titanium blanks and original machinery are still available. The main benefits of converting an EW tankhouse into an ER circuit is that copper production can be maintained with minimal investment. Adequate anode copper melting and

9 casting equipment must be available close enough to the refinery to receive revert anode scrap Full use can be made of the converted tankhouse. All copper is produced at LME Grade A; probably 99.99% copper An ER facility will be obtained for a significantly lower cost compared to the new as built cost. FARADELK Faradelk is a group of highly experienced hydro and electrometallurgical designers all with strong backgrounds from major engineering and construction companies. Faradelk brings these individuals together to pool their respective experience, knowledge resources and data bases for a very focused service the production of high quality level feasibility/conceptual studies for clients with small to medium sized projects and the review/auditing of studies for financial and lending institutions. The range of technologies covered is restricted to copper leach/sx/ew, copper electrorefining, zinc, cobalt and nickel SX/EW and uranium SX both as direct SX or with CIX /SX We prefer to do a small range of subjects as the world s best. The production of conceptual/feasibility studies is often performed by the in-house engineering capability of large mining and metals companies. However, sometimes such groups lack E&C know-how and experience and in many cases smaller mining companies do not have such engineering staff. The initial investigations for projects do not need the cumbersome, high overhead, procedure riddled inefficiency of large organizations. Our experience in large E&C companies has demonstrated many times that such studies can be produced to near basic engineering level at down to 30% estimate accuracy by small teams, provided the team members are all well experienced in this type of work. Faradelk has the experienced people and E&C background to fill these needs. Many years experience has proved that knowing the answer is not as important as knowing where to find the answer. The network of our associates throughout hydrometallurgical and electrometallurgical plant equipment and process suppliers, leach and soils consultants, product marketing groups etc. ensures that the correct parties are consulted for all parts of a project study. Faradelk Ltd., 1 Beechside, Staindrop, Co. Durham DL2 3PE, England Tel faradelk@yahoo.co.uk Web

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